We Fly: Bristell B23 Energic

In 2011, I glimpsed the future, when I first saw an electric airplane fly at EAA AirVenture in Oshkosh, Wisconsin. Fourteen years later, I’ve finally flown an electric airplane for the first time. It’s the Bristell B23 Energic, which is now touring in the U.S. giving demonstration flights.

The airplane that flew at AirVenture was a twin-engine, experimental Lazair ultralight designed by Dale Kramer. Kramer removed the two gas engines and replaced them with electric motors. He opened the wings to show me that he had loaded them with perhaps a hundred or more lithium polymer (LiPo) batteries that were connected together to power the two electric motors.

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I was surprised to learn that the batteries were the same ones sold to power remote-control airplanes. Total capacity of the batteries was 7.2 kW (kilowatt-hours). They weighed 100 pounds and cost $3,300. In one of the early flights, Kramer took off from a lake, flew about 30 feet above the water and was able to fly for a total of 50 minutes. His airspeed, which he measured by timing his upwind and downwind legs over a 5-mile course, was 37 mph. Fourteen years later, an hour is still the target time that most electric plane manufacturers are seeking for a trainer aircraft.

Enter the Bristell B23 Energic, the result of a partnership between Czech airplane manufacturer BRM Aero and Swiss electric propulsion company H55. BRM Aero, which was founded in 2009, produces seven light sport and ultralight aircraft, including the B23 and B23 Energic, and now ships over a hundred aircraft per year. The B23 Energic is based on BRM Aero’s piston-powered B23, a two-seat, all-metal, low-wing design that’s available with either the 100 hp 912iS Rotax engine or the 141 hp 915iS turbocharged version. Fuel capacity is 120 liters (31.7 gallons).

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The piston-powered B23 has an empty weight of 450 kilograms (992 pounds), a maximum takeoff weight (MTOW) of 750 kg (1,653 pounds), and a useful load of 300 kg (661 pounds). It has two wing lockers that can each hold up to 20 kg (44 pounds) of luggage, and a fuselage luggage compartment that can carry 15 kg (33 pounds).

Specifications for the 100 hp 912iS version include a “maximum horizontal flight speed” or VH at FL 120, which I take to be the cruise speed, of 107 ktas. Its never exceed speed VNE is 156 kcas. VS0, the stall speed with flaps extended, is 43 kcas. The best rate of climb with flaps retracted is 702 fpm.

The B23 Energic uses the B23’s airframe but replaces its engine with H55’s electric motor and batteries. H55 is a leader in electric propulsion systems for aviation. It was founded by former Solar Impulse team members. The Solar Impulse 2 was a solar-powered aircraft that flew around the world in 2015-16. It boasted a wingspan wider than a Boeing 747, and it was covered with over 17,000 solar cells that charged its batteries in the daytime, allowing it to fly through the night.

The longest leg of the journey, which was from Hawaii to Moffett Field (KNUQ) in Mountain View, California, lasted five days and five nights and is one of the reasons the company is called H55. Another is that the company was founded in a hangar named Hangar 55.

In addition to electrifying the B23, the company has several strategic partnerships to install its components and batteries in other aircraft. For example, the company is working with Pratt & Whitney Canada to develop the energy storage system for a hybrid-electric, 49-seat de Havilland Canada DHC-8 Q400 demonstrator.

It’s also working with CAE, a flight simulator provider with a network of flight schools, to convert its fleet of 80 Piper Archer aircraft to electric power. For that project, H55 will provide a larger version of the battery pack used in the B23 Energic. Safran, a French multinational aerospace and defense company, will supply the electric motor.

That work is being done as a supplemental type certificate (STC) that will be held by CAE. Long term, that provides a potential path for Piper to offer an electric version of the Archer and for Archer owners to convert their planes to electric power.

H55 is also working with Harbour Air, a Canadian company known for having the world’s largest all-seaplane fleet. It provides scheduled service between a number of locations in the Pacific Northwest and also offers scenic tours. The project is similar to the one with CAE, in that Harbour Air will hold the STC to convert its fleet of de Havilland DHC-2 Beavers to electric propulsion. H55 will provide the battery modules and magniX will provide its magni650 electric engine.

The Bristell B23 Energic in flight. [Courtesy: Ronald Rugel Photography]

The Basics

When I first saw the B23 Energic, it reminded me of the now discontinued PiperSport. That was Piper’s rebranded version of the Czech-built CZAW SportCruiser, which I test-flew at the Piper factory in 2010. It’s my favorite light sport aircraft (LSA) that I’ve flown to date because of its excellent visibility through the front-hinged bubble canopy and well-balanced center stick that made it easy to fly. Later I learned that the B23 and the CZAW SportCruiser were both designed by Milan Bristela, who is the founder and CEO of BRM Aero.

The B23 Energic replaces the B23’s engine and fuel tanks with the H55’s modular electric propulsion system (EPS). Power is provided from a 600-volt, 48-kWh , lithium-ion battery pack that’s built into the wings. To put that into perspective, the average U.S. household uses approximately 29-30 KWh per day.

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The current battery consists of 86 modules, each of which contains 36 4.2-volt cells. The plan for the production version is to move to 120 cell modules to reduce cost and increase power density. Total weight of the batteries is about 300 kg (660 pounds).

The battery is guaranteed to last for at least 1,500 charge cycles. If each flight were to average an hour, you might think of that as similar to a 1,500-hour engine TBO, though most engine TBOs are closer to 2,000 hours. No word on what it will cost to replace the batteries, though company cofounder André Borschberg points out that when the batteries are replaced, newer battery technology will be available that will likely increase the aircraft’s endurance.

The maximum takeoff weight of the piston-powered Bristell 23 is 750 kg, but the current test prototype of the electric version has a maximum takeoff weight that’s 100 kg higher at 850 kg, which is 1,874 pounds. The team is seeking a 50 kg increase in gross weight for the final production version, bringing it to 900 kg, or 1,984 pounds. If the company gets the max gross weight increase, it doesn’t plan to increase the payload, as all of that weight gain will be used for more batteries.

The aircraft’s endurance is specified at 60 minutes, plus 10 minutes of energy reserve. That just reaches the industry’s target for a practical flight trainer, though it relies on a difference between European Union Aviation Safety Agency (EASA) and FAA rules for fuel reserve, or in this case, energy reserve.

In the U.S., the FAA’s § 91.151 says that no person may begin a flight in an airplane under VFR conditions unless there is enough fuel to fly to the first point of intended landing and, assuming normal cruising speed, for at least 30 minutes in the day and at least 45 minutes at night. There is a relatively new exception for eVTOLs.

Per SFAR 120, titled “Powered Lift: Pilot Certification and Training; Operational Requirements,” a 20-minute reserve is permitted provided they are capable of landing in vertical lift mode along their entire route. But there is no similar exemption for electric airplanes, so at present they must follow 91.151’s 30-minute rule.

The 104 kW motor uses three separate windings to provide redundancy, should one fail.
The current battery consists of 86 modules, each of which contains 36 4.2-volt cells. [Courtesy: Jim Koepnick]

By contrast, EASA’s AMC1 NCO.OP.125(a) (for noncommercial operations) sets less stringent minimum final reserve energy requirements for all airplanes, including electric ones. That rule says for airplanes that the final reserve fuel (FRF)/energy should be no less than the required fuel/energy to fly: “for 10 minutes at maximum continuous cruise power at 1,500 feet (450 meters) above the destination under VFR by day, taking off and landing at the same aerodrome/landing site, and always remaining within sight of that aerodrome/landing site.” The 1,500-feet requirement is not a required flight altitude, but is a reference altitude for energy consumption calculations.

So European operators will be able to fly the current version of the B23 Energic for 60 minutes. But at the moment, if the current version of the airplane were to be flown in the U.S., it could only fly for 40 minutes, in order to land with a 30-minute energy reserve. That’s why H55 plans to complete certification in Europe first and begin shipping the aircraft to customers there in 2026. It doesn’t expect to start shipping to U.S. customers until 2027, at which point the company expects to have a larger battery capacity, so the aircraft can still achieve something close to a 60-minute endurance with a 30-minute reserve.

The B23 Energic will ship with a three-blade, fixed-pitch, carbon-composite propeller made by DUC Hélices. The pitch is adjustable on the ground by loosening some nuts and twisting each prop blade to a new pitch angle. However, the airplane won’t have the wing lockers or the optional BRS parachute found in the piston-powered version. Both were eliminated to increase the weight available for batteries.

One nice feature of the airplane is its generous cabin width of 51.2 inches. I’m just over 6-foot tall, and the cabin felt roomy, not at all like the Cessna 150 that I learned to fly in many years ago. The wingspan is 30.4 feet and length is 21.6 feet, so the aircraft will easily fit into a standard T-hangar.

One area in which the B23 Energic is more limited than the piston-powered B23 is in payload. The maximum payload for the Energic is 180 kg, or 397 pounds. That’s 73 pounds less than the piston-powered version loaded with full fuel.

Luckily for me, earlier this year my doctor told me to lose weight, and I was down to 200 pounds, which, coincidentally, was the maximum weight that H55 allowed for passengers for demo flights.

In most aircraft, pilots are used to trading fuel for payload, depending on which they want to carry more of. But in electric aircraft, the battery weight is fixed, so there’s no way to offload some batteries to increase the payload.

But electric planes do have the advantage over pistons when operating at high density altitude. Normally aspirated engines lose power in high, hot, and humid conditions. However, electric motors still produce the same power regardless of atmospheric conditions. So electric training aircraft should be relatively more attractive to flight schools that operate in high or hot locations.

The big advantage of electric aircraft is their low, hourly operating cost. Depending upon the local cost of electricity, I was told that B23 Energic operators could expect to pay about $8-$9 to operate the aircraft for an hour. The cost of fuel and oil for a typical trainer aircraft probably costs at least $50 per hour, so that’s a big difference.

Flying It

My demo pilot was Laurent Wülser, who grew up in Switzerland and has been the president of the Aéroclub de Genève for the past 18 years. Wülser let me do all of the flying around Palo Alto Airport (KPAO) in California, and I sat in the right seat which, as a CFI, is where I spend most of my time.

He talked me through the electrical switches, which included separate ones for the batteries in each wing and for the cooling and backup cooling systems. The electric motor is liquid cooled, and the battery modules are air cooled.

An 800-volt charger is used, but charge time varies with environmental conditions and charger performance.
The motor management computer (MMC) is above the power management computer (PMC). [LEFT - Courtesy: Jim Koepnick; RIGHT-- Courtesy: Tamar Burton] — An 800-volt charger is used, but charge time varies with environmental conditions and charger performance.
The motor management computer (MMC) is above the power management computer (PMC). [LEFT – Courtesy: Jim Koepnick; RIGHT– Courtesy: Tamar Burton]

The panel in our FLYING review aircraft, which had the Swiss call sign HB-SXD, included two Garmin G5s, which act as small PFDs, displaying the flight instruments. The only large glass displays were the motor management computer (MMC) and power management computer (PMC). The latter display displayed a placard that read: “Single battery operations limited to 150 amps.” As an electrical engineer, I can assure you, that’s a lot of juice. Later Laurent showed me that the PMC displays the voltage and current for the left and right batteries, and the total energy left in the battery, which was 43 kWh for our flight.

The throttle lever is mounted on the center console between the pilot and copilot seats. Power is set by moving it to the desired setting in kilowatts, not manifold pressure or rpm. Laurent said we would be limited to 95 kW for takeoff power in the aircraft we flew. Next to the throttle is a guarded toggle switch that is the backup throttle. It has two positions—one for cruise at 40 kW and the other for 80 kW.

Start-up consisted of turning on the cockpit battery switch and verifying that we had a minimum of 13.1 volts. Laurent then turned on the left- and right-wing batteries and verified that the motor was above the minimum minus-10 degrees Celsius (14 degrees Fahrenheit) starting temperature. He set the EFU switch to ready, confirmed the inverter was online, and then switched it to fly, at which point the prop began turning at 200 rpm. He then advanced the throttle to 10 kW to check the power and rpm. We then headed to the runway, as no further run-up was required. Taxiing was easy, as the airplane has a steerable nosewheel.

The day I flew, H55 was looking to get three, and sometimes four, demo flights per battery charge. So, our flight was a single loop around the traffic pattern. Laurent suggested we do a static run-up. I didn’t advance the throttle far enough, so he eased it up to takeoff power.

I rotated at 60 knots and targeted a climb speed of 70 knots. Once established in climb, we reduced the power to 70 kW. During the climb, I raised the flaps, and there was virtually no change in pitch, which was a nice difference from other popular planes I fly.

At traffic pattern altitude, we reduced power to 40 kW. On downwind, the tower approved a left 360, which gave me a little more flying time. Just a light touch of the center stick got us into a smooth turn. I found the controls to be light and well balanced.

The maximum flap speed is 80 knots. I reduced power to about 20 kW for the descent, and briefly found myself overspeeding the flaps. Laurent said that’s not uncommon until full flaps are deployed. He suggested we fly the pattern at 70 knots and reduce to 60-65 knots on final.

Landing was straightforward. I did find myself low on final and I corrected by adding some power. I left some power on until I was in the flare, since I didn’t know how quickly the plane might drop when I pulled the power. I was able to get the nose up relatively high before we touched down, and it was a great landing that was easy to accomplish.

After landing, I realized that once we took off, I totally forgot that we were flying an electric aircraft. And that’s probably the best testimonial for electric propulsion: It should be an enabling technology and not get in your way. Electric aircraft are now poised to start contributing to general aviation, and I can’t wait to fly another one.